Surgical navigation (surgery assistance information display) has been performed in the past for displaying the proper position of the distal end of the surgical instrument on a CT (computed tomography) or MRI (magnetic resonance imaging) image to assist a surgeon when an endoscope or another surgical instrument is inserted into a patient's body. For example, Patent Document 1 describes a surgery assistance system proposed by the present inventors, which is a technique for aligning a 3-dimensional surface shape of a patient measured by a 3-dimensional shape measurement device with 3-dimensional tomographical data imaged in advance. A technique is also described whereby a 3-dimensional shape measurement device for measuring the 3-dimensional surface shape of a patient is used to measure an indicator part (sphere 12 in FIG. 1) for position and orientation detection that is attached to the surgical instrument, and the position and orientation of the surgical instrument is computed. However, these methods are merely for displaying the position of the distal end of a surgical instrument, pointer, or another instrument, and do not indicate the portion of a preoperative CT or MRI image to which a site imaged by an endoscope corresponds.
If it were possible to confirm the area of a preoperative CT or another image to which a site imaged by an endoscope (surgical area displayed on the monitor of an endoscope) corresponds, then, for example, a surgeon could freely take any surgical instrument in the right hand and continuously operate while using an endoscope held in the left hand to confirm the operated area by direct visual observation and recognize which area of a preoperative CT or another image is being observed.
Patent Documents 2 and 3 describe such conventional techniques whereby a site imaged by an endoscope is displayed on an image. Techniques proposed in the past by the present inventors include Japanese Patent Application No. 2007-22077 (not yet disclosed). Patent Document 2 describes a technique whereby the direction of the optical axis of a rigid endoscope in use is displayed on a 3-dimensional tomographic image in a surgery navigation device.
Patent Document 3 describes a technique whereby the location observed by an endoscope is determined and displayed on a preoperative CT/MRI through the use of an endoscope having distance measurement means (triangulation or ultrasonic sensor or the like using spotlight irradiation) for measuring the distance from the distal end of an insertion part of an endoscope inserted into a patient's body to an operation site within the patient's body.
In Patent Documents 2 and 3, a luminescent element or another marker and a position sensor for detecting the marker are used to detect the position and orientation of the endoscope, but these systems require that some type of marker be attached to the patient, or that a device be separately provided for measuring the shape of the patient in order to align the 3-dimensional tomographical data with a coordinate system for the patient, thus inconveniencing the patient and increasing the complexity of the system.
However, in the conventional technique of the present inventors described in Japanese Patent Application No. 2007-22077 (not yet disclosed), a 3-dimensional shape measurement device for measuring the 3-dimensional surface shape of the patient is used to detect the position and orientation of a rigid endoscope, and it is possible to prevent inconvenience to the patient and increased complexity of the system.
However, in Japanese Patent Application No. 2007-22077, the optical axis of the endoscope is assumed to be a nominal value, the same as in the techniques of Patent Documents 2 and 3, and calibration of the optical axis of the endoscope is not addressed. In a straight-view endoscope, for example, the optical axis information of the endoscope is displayed under the assumption that the optical axis of the endoscope passes through the center of the endoscope barrel, i.e., the optical axis has a nominal value such that the angle formed by the endoscope optical axis and a line through the center of the endoscope barrel is 0 degrees.
Since the objects viewed by an endoscope are usually relatively close to the lens, no consideration has been given to calibrating the optical axis of the endoscope, although calibration of lens position and other factors has been considered. In Patent Document 4, for example, a device is described for calibrating the lens position and field of view of an endoscope having a long shaft and a distal-end lens. However, the field of view of the endoscope is adjusted by displaying an existing image in the endoscope and adjusting the displayed image, and no description or suggestion is given of calibrating the optical axis of the endoscope.    [Patent Document 1] Publication of Unexamined Japanese Patent Application No. 2007-209531    [Patent Document 2] Publication of Unexamined Japanese Patent Application No. 2001-293006    [Patent Document 3] Publication of Unexamined Japanese Patent Application No. 2001-204738    [Patent Document 4] PCT (WO) 2003-528688